The invention relates to a process for recycling of synthetic plastics material containing gas, for example foamed polystyrene, in a plant in which synthetic plastics material is molten, filtered, degased, granulated and mixed with gas again, this gas re-introduction being performed in the same plant as the granulation. Further, the invention relates to an apparatus for performing such a process.
The usual process steps when re-introducing gas into foamed synthetic plastics material, in particular polystyrene, consist in that the synthetic plastics material is comminuted in a tearing and densifying apparatus and is then plasticized in an extruder and de-gased. The so produced plasticized, de-gased synthetic plastics material is worked up to granules in a granulating apparatus and is solidified thereby. The granulate forms a pourable homogeneous, de-gased mass which then is introduced again into an extruder and is molten therein. During the extrusion which can be performed by means of extruders comprising one or more extruder screw, a gas is fed in a controlled manner to the extruder wherein gas is introduced within the extruder into the synthetic plastics melt and is homogenized therewith, considering the necessary holding time in the extruder. When doing this, the use of a pourable homogeneous granulate is compulsory necessary because only in such a manner a continuous flow of the melt in the extruder and therefore a continuous filling degree of the foaming gas in the melt can be ensured.
A similar process has become known from the document "Plastver-arbeiter", volume 42, 1991, number 6, pages 124, 125. This document suggests to mill packages of polystyrene and subsequently to heat this material in a roller extruder so that the material is de-gased and plasticized. The thus obtained solidified final product, namely cristallized polystyrene, is then foamed again in an oxtruder.
These known processes have the disadvantage that two melting steps are necessary which, as a rule, have to be carried out at different places, because the homogeneous introduction of the gas is a critical thing and, therefore, cannot be made everywhere. Therefore, as a rule, the synthetic plastics material to be recycled is granulated there where an apparatus for comminuting and densifying the material is at disposal. The granulate is then sent to a plant where the material is provided with a gas content again. This involves costs for transport and transformation which lead to a high price of the recycled and gas-containing material so that the thus obtained recycling product can hardly be sold. The consequence is an environmental load by foamed synthetic plastics material scrap or by improper processed synthetic plastics material. In this connection it is of detrimental influence that in the past frequently such gases have been used as foaming gases (for example fluorocarbons) which are critical for environmental reasons and, therefore, should not get into the atmosphere.
It is also known to melt and degas scraps of foamed thermoplastic synthetic plastics material in an extruder comprising several worms. In parallel to this extruder a further extruder is provided in which thermoplastic particles, hence new material, are molten and mixed with gas. The outlets of the two extruders are combined to a mixing and cooling station to which a further extruder as well as a mould are connected, in which the material is foamed. Finally, the foamed material is cut to the desired shape or is granulated. For performing this process, a considerable effort in apparatus and space is required and the above described disadvantages can not fully be avoided by this process, because the granulate of new material must at first be produced and then be stored so that the enthalpy is lost which was in the new material until its granulation.
The invention has as its object to improve a process of the last described kind so that the recycling process of the gas-containing synthetic plastics material can be effected quicker, simplier and requiring considerably less effort in apparatus and space and without environmental load and without the use of new material. The invention solves this task in that the gas is fed to that synthetic plastics material which was de-gased and this before its granulation in such plasticized condition of the synthetic plastics material which originates from the melting process and the degasing step, and that for homogenisation of the gas content the gas volume introduced per time unit into the synthetic plastics material and the mixture volume processed per time unit by the mixer are controlled proportionally to the synthetic plastics material volume supplied per time unit to the gas-introduction step. For performing the inventive process, therefore, only one single plant is required and comminuting, plasticising, filtering, re-gasing and solidifying of the processed synthetic plastics material can be effected at the same place and, therefore, avoiding transport costs. Within this, the known tearing and comminuting apparatus can be used, which must only be provided with a device for the introduction of the gas and with a device for the subsequent mixing and solidifying, in order to be able to perform the inventive process. The plant expenses required for this are comparatively low and amortize already within a short time by the saved transport costs and charge fees for the extraneous device for re-introduction of the gas. The quality of the obtained final product is good, also with respect to homogenity of the foaming gas content, what is obtained by the inventive control of the gas introduction in dependence from the amount of flow of the synthetic plastics material supplied to the gas introduction step. A further advantage of the inventive process consists in that the processed synthetic plastics material must be molten one single time only so that the second melting process required hereuntofore can be saved. This means also, that the synthetic plastics material is treated more gently because each melting process is combined with the danger of a reduction of the molecule chain length of the synthetic plastics material. Further, when performing the inventive process, the synthetic plastics material must be solidified one single time only, and this--in contradiction to the initially described known processes--after introduction of the gas only.
In contradiction to the last described known process, within the inventive process only such synthetic plastics material is processed which was subjected to the recycling process. Therefore, the use of new material and therefore the expenses for material, transport, storage and melting connected therewith can be saved. Further, the inventive process requires substantially less effort in apparatus, when compared with the known processes.
The invention considers also that the synthetic plastics material subjected to the recycling process is not always of the same quality, in particular with respect to the specific weight or, respectively, to the foaming degree. The invention considers that, when material of lower weight (higher foaming degree) is processed, the output of the apparatus used for the melting process will decrease. As a consequence thereof, the synthetic plastics material amount supplied per time unit to the gas introduction step, decreases. If this volume measured per time unit is used in the sense of the invention as the command variable for the gas amount used per time unit for being added to the synthetic plastics material, the gas content for the final recycling product remains at least substantially constant with what is desired with respect to the further processing of this final product. The homogenity of this final product is also enhanced by the proportional readjustment of the mixture amount worked up per time unit by the mixer, in dependence from the mentioned command variable, because undermixing as well as overmixing are avoided and thus the cell structure of the regenerate obtained as the final product is kept at least substantially constant.
Since as a rule it is desired to operate the plant with an output as high as possible or, respectively, as far as possible to fully utilize the capacity of the present plant, according to a preferred embodiment of the inventive process, the volume of the synthetic plastics material supplied per time unit to the gas introduction step is kept within a predetermined interval, preferably near the maximum capacity of the plant used. This can easily be obtained by operating the plant elements used for plasticizing and de-gasing of the supplied synthetic plastics material quicker or, respectively, with an increased output.
As a rule, a pump for the synthetic plastics material flow supplied to the gas introduction step is inserted between those plant elements that serve for plasticizing the supplied synthetic plastics material, and those plant elements that serve for the re-introduction of the gas. According to a preferred embodiment of the inventive process, the supplied synthetic plastics material is comminuted and then is plasticized and de-gased by means of a screw, that the so obtained de-gased melt is supplied by means of a melt pump, preferably a gear pump, to a mixer to which also the gas used is supplied, and that the synthetic plastics material amount delivered per time unit by the melt pump is measured and is used as the command variable for the gas supply, and that the pressure of the synthetic plastics material on the suction side of the melt pump is measured and is kept by varying the speed of the melt pump and/or the speed of the worm within a predetermined interval, preferably at a value that is as constant as possible. Therefore, the melt pressure before the melt pump is kept almost constant by adjusting the delivery of the plasticizing screw or the melt pump, and by proportional readjustment of the gas supply in dependence on the delivery per time unit of the melt pump used as the command variable, the desired constant gas content in the final product produced by the mixer is obtained. Keeping the pressure of the material supplied to the melt pump within a predetermined interval ensures that the melt pump does not make idle strokes, and that therefore the volume delivered is exactly proportional to the run of the melt pump, in particular to the speed of the gear pump. Further, in such a manner the plant can easily be operated near its maximum capacity.
Within the spirit of the invention, it is particularly favourable to add the gas to the flow of the synthetic plastics material already before its introduction into the mixer, in order to obtain a certain mixture between the synthetic plastics material and the foaming gas already before the synthetic plastics material or, respectively, the gas enters the mixer.
For a uniform absorption of the gas, the melt pressure behind the melt pump is of importance. According to a further embodiment of the invention, therefore, the process is conducted so that when the pressure of the synthetic plastics material melt supplied by the melt pump deviates from a predetermined desired value, the viscosity of the mixture prepared by the mixer is correspondingly changed by cooling and/or heating, preferably until the desired value is reached.
As a rule, pentane is used for the gas that is again introduced. In order to avoid an explosion in the plant section behind the degasing step, within the spirit of the invention the vacuum is controlled that is used for degasing the synthetic plastics material.
The inventive apparatus for performing the inventive process starts from a plant having a comminuting means for the synthetic plastics material to be processed, to which means a motor-driven screw for plasticizing this material is connected, a de-gasing means for the synthetic plastics material being related to that screw. Starting therefrom, the inventive apparatus is characterized in that the outlet of the screw is connected by means of at least one line to a mixer to which also a device for supplying gas is connected, and that to at least one of these lines a device for measuring the synthetic plastics material amount flowing per time unit in this line is connected, which device is connected to a control unit for the gas supply to the mixer and for the speed of the motor of the screw. In such a manner the inventive process can be performed with low constructional effort. Since, however, the pressure in the outlet of the screw and, therefore, in the line leading to the mixer alone will frequently not be sufficiently high in order to reliably supply the plasticized synthetic plastics material into the mixer, according to a preferred embodiment of the inventive apparatus, a melt pump, in particular a gear pump, for supplying the synthetic plastics material melt is inserted into the line leading from the screw to the mixer, a measuring device for the speed of this melt pump being provided, noting that to this line a pressure sensor for the pressure at the suction side of this melt pump is connected, which pressure sensor, as well as the measuring device, are connected to the control unit. The, suitably electronic, control unit calculates the measuring results obtained from the pressure sensor and from the measuring device and controls in correspondence to these measuring results the gas supply to the synthetic plastics material melt in such a manner that the gas content thereof is kept at least substantially constant so that a homogeneous final product is obtained.